linux-kernel - Re: PATCH - x86-64 signed-compare bug, was Re: select() setting ERESTARTNOHAND (514).

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Date:	Thu, 11 Jan 2007 20:38:03 +0100
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	Neil Brown <neilb@...e.de>
Cc:	Andi Kleen <ak@...e.de>, Sean Reifschneider <jafo@...my.com>,
	linux-kernel@...r.kernel.org
Subject: Re: PATCH - x86-64 signed-compare bug, was Re: select() setting ERESTARTNOHAND (514).

On Thursday 11 January 2007 02:02, Neil Brown wrote:
> If regs->rax is unsigned long, then I would think the compiler would
> be allowed to convert
> 
>    switch (regs->rax) {
> 	case -514 : whatever;
>    }
> 
> to a no-op, as regs->rax will never have a negative value.

In C, you never actually compare different types. They always
promoted to some common type first.

both sides of (impicit) == here get promoted to "biggest" integer,
in this case, to unsigned long. "-514" is an int, so it gets
sign extended to the width of "long" and then converted to
unsigned long.
--
vda
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